Because of position. What if someone is color-blind? Or uses a seeing-eye dog? The position of the lights is always the same. Red is always on the top of vertical lights, and on the left on horizontal lights.
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Rocket HazmatApr 29 '14 at 19:06

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It's worth noting here that the UK railwaysdo now use light signals where red, green and yellow can all use the same light (using LED technology).
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Andrew LeachApr 29 '14 at 21:11

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@AndrewLeach - are those signals facing train drivers and other railway staff? I would hypothesise one can't get a job driving a train or maintaining tracks if one is visually impared
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Toni LeighApr 29 '14 at 21:56

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@RocketHazmat, agree on color-blindness. But seeing-eye dogs don't use stop lights; not by position, not by color. That's an urban legend probably fostered by TV and movies. The handler of the dog (the blind person) has to determine when it's safe to cross based on either an audible crossing signal or the sound of traffic patterns, and then command the dog to go forward. The best a dog can do is refuse to go and/or cross in front of the handler and block the way, which they are trained to do when they perceive an imminent danger.
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Euro MicelliApr 30 '14 at 4:10

8 Answers
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In London, England in 1865 there was a growing concern over the amount of horse-drawn traffic causing danger to pedestrians trying to cross the roads. A railway manager and engineer named John Peake Knight, who specialized in designing signaling systems for the British railway, approached the Metropolitan Police with the idea of using a similar semaphore/lighted system for road traffic. In the daytime, this semaphore method used an arm or arms that could be raised or lowered by a police officer, notifying carriages when they should stop when the arm(s) stuck out sideways. At night, his system used the red and green colors to indicate the same type of thing, again traditional to the railroads.

As traffic signaling evolved the use of lights or flags continues. From the same article:

In 1920 in Detroit Michigan, a policeman named William L. Potts invented the four-way, three-color traffic signal using all three of the colors now used in the railroad system. Thus, Detroit became the first to use the red, green, and yellow lights to control road traffic.

In 1920, there would be little option other then to provide 3 independent lights to show 3 different colors. While other options might have existed, there complexity may have limited their adoption.

Technology & Economics

To specifically call out the reason above: because of technology. Today's technology allows for amazing things, but to think that a single-light traffic signal would be both technologically and economically feasible in 1870, 1920, 1980 (or even 1990) is a very hard argument. You can argue the ability in the later years, but can you argue the economics?

Technologically it is very easy to put 3 lights in a row and tell them when to turn on or off. Economically it is very easy to put 3 lights in a row and tell them when to turn on or off... and fix them when they break.

The technology to do it "this way" vs "that way" is simply a lot cheaper.

Color Blindness

EDIT: Some comments on this answer were not taking this section in the context of historical precedence. I have attempted to clarify the points being made, in the greater context of my final answer at the end. I have also corrected some information on standards which where inaccurate.

While "because of color blindness" is a popular belief of the historical, this is simply false. Looking back at the historical reasons, for the time when traffic signals were being developed, the idea of catering to a rarely known condition would be unheard of.

A two color light, based on the railways, was first introduced in 1912 (per Wikipedia):

The first electric traffic light was developed in 1912... It had two colors, red and green, and a buzzer.

All of these different types of lighting systems began to present a problem. Drivers could drive through different areas and encounter several different types of systems, causing confusion and frustration. So, in 1935, the Federal Highway Administration created “The Manual on Uniform Traffic Control Devices.” This document set uniform standards for all traffic signals and road signs.

Note that these standards were put in place for the benefit of everyone! This was not an action taken on behalf of "color-vision deficient drivers". No mention of such a population is made in the 1935 Manual on Uniform Traffic Control Devices (p. 71).

The idea that the order (Top = Red, Middle = Yellow, Bottom = Green) is for color blind individuals is quickly debunked when you search for "sideways traffic lights".

... or go someplace like Syracuse, New York:

Order is common, and mostly followed, but standards are not always followed and the original development of the traffic signal pattern was not guided by the standards that exist today.

There were also no standards in traffic light shading of the red/yellow/green colors - meaning one traffic light may use shades that allow certain color blind individuals to distinguish red vs. green, but another may not. While standards exist today, these did guide the use of these colors or their shades during early development.

Color blindness does not mean you can not see color (except for the very rare "achromatopsia" condition) - it is possible to provide shades and luminosity that do work for the majority of color blind people.

But it does help! Certainly the positional nature helps colorblind people, as long as they know what order the lights are in. This is very likely a reason why a single-light concept would be very difficult to implement, given today's attitudes and awareness of safety (for all drivers). However, the point here is that color blindness is not a reason why they were made that way in the first place.

So...

Historically they were made that way; improving over time until the technology to create and maintain them became less and less; creating a situation where the economics to change them are prohibitive.

But... User Experience!

Yeah, I know - it's a UX site. Sometimes things can't be explained in terms of UX. Sometimes things exist simply because they have existed that way for so long.

Thank you, thank you, thank you for dissecting the color blindness argument. "Because color blindness" is the most overused UX justification I see on this site, often being dragged out by people who know nothing about it.
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Joshua BarronApr 29 '14 at 22:20

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According to this article the upside-down traffic light in Syracuse is a very special case, controversial, and accompanied by special warning signs. I don't think you can claim it as evidence that colorblindness is not a consideration here; it appears more likely to me that it is a consideration, and the officials in Syracuse consciously chose to disregard it. As for the sideways lights, I think you'll find that in such cases, red is always on the left.
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Nate EldredgeApr 30 '14 at 2:49

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In Belgium the law explicitly (Dutch link) states that the light have to be 3 colored and red has to be on top and green at the bottom. Thus to change the lights they'd first have to change the law.
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ratchet freakApr 30 '14 at 9:01

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@josh3736, while standards have been created concerning the positioning & color this was not done until 1935, for everyones benefit (not explicitly for the "color-vision deficient"); luminosity and chromaticity not even later. As I point out in my closing statement the positioning does help color blind individuals, but there original creation was not guided by the condition.
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Evil Closet MonkeyApr 30 '14 at 19:43

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There is another reason for more than one bulb: failing bulbs. When you have three separate bulbs in two out of three cases you know exactly what situation you are in (as one of the unbroken bulbs is on) and you can thus also spot the broken bulb and know that it would be on (because the others are off as well) - you would only have to wait at most one light change to know which bulb is broken (assuming that the chance of all three failing at the same time is slim).
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Marjan VenemaApr 30 '14 at 20:16

One reason for the three lights, in the UK at least, is that you have many states. Two of the lights can be on at one time, i.e. red and amber signalling that you should get ready to go, where as a single amber light would mean get ready to stop. The concept you link to doesn't cover this use case.

There are also historic reasons for the 3 light traffic signal, they used to be operated my humans and originally were gas burning lamps (briefly) - this was before LEDs even existed, so they needed 3 states. Traffic lights all started on the railway, the amber light was a signal to slow down and give the train driver advanced notice it's state was about to change.

With the specific implementation you mentioned, I can also see a few other use cases (on UK roads) that would be rather difficult to implement - traffic signals do more than tell people when to stop and go.

A few examples of other uses are:

@Steve Wortham also makes a great point that a moving light (much like something animating on a web page) is more likely to catch your attention than a single light changing colour, especially if you are not paying complete attention.

Another reason why we may not have changed the design so much could be that - if we were to change the traffic light system, it would cost a HUGE amount of money for a government to not only implement the lights themselves but to also teach and educate road users of this new system.

+1 for mentioning getting ready to stop and getting ready to move, this is the only reason for having the middle light, also I would add that changing a system that works well and is established, even for one that could be a slight improvement, would introduce more problems than it would fix
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Toni LeighApr 29 '14 at 21:58

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Switzerland requires drivers to stop the engine at traffic lights and railway crossings, if they're at least 3 cars behind. The red-yellow transition to green is very important in this case, as starting your engine again takes a second or two.
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FuhrmanatorApr 30 '14 at 1:47

The concept is an array of LEDs, so it does cover other use cases. It could display half green / half yellow for the intermediate state, or form shapes like arrows.
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nmcleanApr 30 '14 at 15:21

Thanks for mentioning arrow lights--if there are four lights arranged vertically, it's likely that a protected turn is possible, which can be good information for a driver or pedestrian to have.
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Milo PriceApr 30 '14 at 20:13

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There is another reason for more than one bulb: failing bulbs. When you have three separate bulbs in two out of three cases you know exactly what situation you are in (as one of the unbroken bulbs is on) and you can thus also spot the broken bulb and know that it would be on (because the others are off as well) - you would only have to wait at most one light change to know which bulb is broken (assuming that the chance of all three failing at the same time is slim).
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Marjan VenemaApr 30 '14 at 20:17

I think a big reason must be that it's easier to see that the light changed (even out of your peripheral vision) when one light goes off and another goes on in another position. The fact that the position AND color of the light is changing makes it easier to notice.

A few answers have made the point that it would be difficult for colour blind users to see the signal. But the truth is, we are all colour blind at least in our peripheral vision.

From your eye's fixation point, at 30 degrees you start to lose trichromatic vision. In other words, you start to lose the ability to differentiate colour. At 50 degrees, you are essentially colour blind. The colours that you see right now in your brain aren't actually 'seen' -- your brain has coloured in the gaps for you.

However, your eyes are good at detecting movement in these extremes. Our response is to saccade to movements in our visual field so we can see them in more detail.

With a single-bulb system, we might be unable to see a change from green to red because, in our peripheral vision, nothing has changed. So it is not only colour that is essential to the signal, but the movement of bulb position which prompts us to fixate on the signal.

The accepted answer contains some good information about the history of traffic lights, but it is dead wrong about the color blindness issue.

First, some facts:

There is (in the US) a legal standard for traffic light color. The MUTCD requires signals to comply with ITE standards that specify luminosity and chromaticity of "red", "yellow", and "green". Indeed, the "green" color has a slight blue hue to aid people with red/green colorblindness.

The order of the colors is also specified by the MUTCD for both vertical and horizontal alignments. Red is always on top (vertical) or the left (horizontal). That one intersection in Syracuse is a curiosity, and technically not legal.

Q: How do the standards for traffic signals accommodate the needs of color-vision deficient drivers? Would it be better to add shapes to the signal indications to help the colorblind identify which signal is on?

A:Distinguishing reds from greens is by far the most common problem for color vision deficient people. This has been taken into account in the MUTCD, which since the 1930s has specified the relative positions of the different colored signal indications, with red always being above yellow, which is always above green (in a vertically-arranged signal.) This positional requirement also applies to arrow indications, with red arrows always being above yellow arrows, which are always above green arrows. For horizontally-arranged signals, similar positional requirements are established, with red always being the left-most indication, etc. It would seem logical that shapes could potentially provide additional assistance to colorblind road users, beyond that already provided by the positional standards. However, a Canadian province that has used such a shape system in its traffic signals in the past has abandoned it due to lack of effectiveness and other issues. Changing the shape of the signal indications results in changes in light output and this impacts how well and how far away the signals can be seen by drivers, particularly under adverse conditions such as fog, smoke, dust, visually complex environments, etc. Therefore, research would be needed to evaluate what changes in signal light sources and intensity might be necessary if the shapes of the lenses were to be changed, and also how colored arrow signals could be associated with shapes. Meanwhile, studies have found that most color blind people adapt and compensate well by using the positional requirements contained in the existing traffic signal standards to help them discern the colors. Also, in the most recent update of the detailed color standards for traffic signals (published by ITE), the green has been specified to be a bit more "blue-green" because this makes it easier for persons with red-green color vision deficiencies to distinguish it from red.

In other words, color blindness has long been a consideration in the design of traffic lights, and it's one of the reasons we keep three separate fixtures arranged in a particular order today.

Some very good additional information, but an incorrect assertion that my answer is "dead wrong". While modern standards exist to benefit color blindness, the historical development of color and positional nature of the lights were not guided by these standards or considerations. Such standards were put in place after and amended to benefit the color-vision deficient population after the fact.
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Evil Closet MonkeyApr 30 '14 at 20:51

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@EvilClosetMonkey Yes, good point. If traffic lights were to be invented now then colour-blindness would likely play a part in the design, but you're right that they're not a new design.
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JonW♦May 1 '14 at 13:27

I knew the laws were there for colorblind people. I just couldn't find it. I was looking in my state laws.
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Darrick HerweheMay 2 '14 at 16:11

Of course, Americans make up less than 5% of the world's population, so American laws and regulations are rather irrelevant in the grand scheme of things.
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BrendonMay 3 '14 at 10:46

I would base American's relevancy on the number of traffic lights which is quite different from using population. I'd guess probably higher given the amount of drivers and cars.
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Michael DurrantMay 4 '14 at 14:28

Do you know the exact reason for having three lamps in a row, instead of one

This question has several parts (at least implicitly).

Why separate single-colour lamps rather than one multicolour lamp?

Why three colours?

Why red, amber and green?

Why are the colours in that order?

Why are they arranged vertically?

The answers to most of these questions arise from the historical development of the system.

Many standards are the result of revision and refinement of a largely arbitrary precursor. It is often expensive (in human lives as well as financially) to change safety-critical conventions that have been learned by large numbers of people, even if there is a theoretically better system.

Origins: Colours.

Some early railroad signals do have several different coloured lights appear at the same single position. They had one lamp. A mechanical arrangement moved coloured lenses in front of the fixed lamp.

Railroad signals were initially semaphore signals where the position of a signalling arm indicated whether it was safe to proceed.

An important point is that, as a form of fail-safe, if the arm raising mechanism fails, gravity will drop the arm to the "stop" position.

OK so road usage followed railway usage, this begs the question - why were those colours chosen by railways? The answer may be that the railways followed established maritime conventions. Ships had red and green lamps on their sides so that other ships could tell which direction the observed ship was travelling. There had to be a convention about which of two converging ships should give way. The convention was (apparently) that if you see a red light, you are required to give way. Another explanation was simply that red, the colour of blood, has long been associated with danger and so became associated with the need to stop.

Why three?

Railroad tracks are divided into sections. Following trains are provided with an indication that a train is on the section ahead (stop/red), has left that section and is in the section ahead of that (caution/yellow) or is much further away (proceed/green).

For night time use, a lamp was added and the motion of the signal arm brought different coloured lenses in front of the lamp.

Order: Failsafe at Red

You can see that with this arrangement, the "stop" indication is at the top position when the line of lenses are vertical. This is a consequence of the fail-safe arm arrangement discussed earlier.

Why separate positions?

With road traffic signals, the semaphore arm was eventually entirely replaced by the lights. The mechanical elements were eliminated, perhaps because they required more maintenance, and three electric lamps were then the simplest signalling system. The colour, number and physical arrangement obviously followed the previous system to avoid confusion.

It is obviously an advantage, where red-green colour-blindness is common, to have a positional reinforcement for the signal meaning.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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Especially since red/green colourblindness is the most common, and the difference between the two can mean life or death at a stoplight. "In individuals with Northern European ancestry, as many as 8 percent of men and 0.5 percent of women experience the common form of red-green color blindness." (Source: Wikipedia)
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Tim FitzGeraldApr 29 '14 at 19:20

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As @EvilClosetMonkey's answer explains, color blindness is definitely not the reason for separating the bulbs of a traffic light. Furthermore, color blindness does not imply you cannot differentiate the red and green colors of a traffic light.
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Joshua BarronApr 29 '14 at 22:21

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@JoshuaBarron especially when many green lights have a blue component to help differentiate between them.
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ratchet freakApr 30 '14 at 8:54

@JoshuaBarron: As various comments point out, the argumentation by EvilClosetMonkey why color blindness is allegedly not the reason, is flawed as it currently stands in the answers.
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O. R. MapperApr 30 '14 at 19:18

@O.R.Mapper, the answer refers to the historical reasoning behind the design of the traffic signal and color blindness not being a defining factor. While modern standards have adapted the historical design to suit color blindness those standards did not define the original design. The answer has since been updated to hopefully be clearer.
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Evil Closet MonkeyApr 30 '14 at 22:13

There have been a couple of passing mentions of trains, where the constraints are interestingly different.

Train signals (in the UK at least, but I don't think they're massively different elsewhere) are also red-amber-green (or on higher-speed lines they're red-amber-doubleamber-green), and they're not consistently one way up; so sometimes the red is at the top, and sometimes at the bottom. This is partly, I understand, because a train driver may have to react to them from further away than a car driver would react to a traffic light, so that they wouldn't necessarily be able to distinguish them based on position, and there is therefore no utility in requiring them to be in a consistent position. It follows (I recall, but can't find a link to corroborate) that train drivers can't be colour-blind.

And just by the way, the reason the four-light sequence is red, amber, and then double-amber (whereas you might initially guess that double-amber is ‘more cautious’ than single amber), is so that it fails safe: if an amber light fails then the signal displays the ‘more cautious’ single amber.

Relevant to the original question, there are indeed now LED-based lights which have all the colours in a single housing, rather than one lamp per colour.